1. Field of the Invention
The present invention relates to a method of controlling transmit power, and more particularly, to a method of controlling transmit power for retransmission packet in uplink dedicated channel.
2. Discussion of the Related Art
In the 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), data transmitted via an uplink enhanced dedicated channel (E-DCH) is first channel coded and multiplexed in a physical channel before being transmitted via an enhanced dedicated physical data channel (E-DPDCH). Within the E-DPDCH frame, data of a plurality of dedicated channels can be transmitted after being multiplexed, and a format of multiplexing E-DCH data of the E-DPDCH is called transport format combination (TFC).
FIG. 1 is an example illustrating a process of channel coding and multiplexing data of E-DCH to E-DPDCH. In the physical layer, the E-DPDCH frame is I/Q multiplexed with a dedicated physical control channel (DPCCH). Thereafter, the multiplexed E-DPDCH and DPCCH is transmitted. The transmission of DPCCH includes various control information, including the TFC information of the DPDCH, uplink power control, and pilot signal for data demodulation.
A transmission power of the E-DPDCH is determined relative to the transmission power of the uplink DPCCH. In the standards of the 3GPP WCDMA, the ratio of the E-DPCCH and the DPCCH transmission powers is defined by a β-factor. The β-factor is further defined by βed, which represents transmission amplitude of the E-DPDCH, and βc, which represents transmission amplitude of DPCCH. The ratio of the E-DPDCH transmission power relative to the transmission power of the DPCCH is determined according to (βed/βc)2. According to the current standard, the β-factor value of (βed/βc) can be determined using two methods.
A first method relates to assigning the β-factor value to each TFC in the upper layer. A second method relates to assigning the β-factor value only to basic TFCs in the upper layer, and regarding other TFCs, determining the β-factor value based on the β-factor values for the basic transport format combinations (TFCs) and other information. In these two methods, the β-factor value is determined as semi-static according to each TFC. Moreover, the relative transmission power of the E-DPDCH is determined based on the DPCCH. A more detailed description on how to calculate the β-factor and the β-factor value from the TFC is explained in the 3GPP standard.
A TFC, which is permitted to transmit in uplink DCH, is determined from the upper layer. A set of TFCs is referred to as TFCS. Subsequently, a TFCS assigned from an upper layer is defined as TFCSRNC. In a physical layer of a user equipment (UE), each TFC is determined and updated to ascertain whether the TFC can actually be used for transmission. This is accomplished by taking into consideration information such as a β-factor value and a maximum transmission power of a UE corresponding to each TFC of the TFCSRNC. After the determination, if a set of TFCs, which have been determined fit to be used for transmission, is called TFCSUE. Thereafter, the UE transmits only to the TFC belonging to both TFCSRNC and TFCSUE.
With respect to Node B scheduling, Node B continuously controls TFCS available for transmission to uplink via enhanced uplink dedicated channel (E-DCH) for each UE in order to maintain a certain level of interference in an uplink direction. For example, if the uplink interference is relatively small, a TFC requiring high transmission power can be transmitted, while if the uplink interference is relatively large, then a TFC requiring high transmission power can be prohibited from transmitting. Through such control, a certain level of uplink interference can be controlled and maintained.
In addition to controlling the TFCS of each UE, Node B can also directly control transmission power permitted to each UE. In other words, Node B execute scheduling to each UE by limiting the transmission power of the E-DPDCH or a ratio of transmission power of E-DPCCH at a specified point. From this situation, a UE can acquire a TFCS, which can transmit within a transmission power level set by Node B, by using a β-factor at each uplink e-DCH transmission point.
With respect to E-DCH, TFCSRNC, TFCSUE, and TFCSNodeB exist. In detail, the TFCSRNC are permitted to transmit by a Radio Network Control (RNC), the TFCSUE are managed by the UE, and TFCSNodeB are controlled by Node B. The UE can transmit data using a TFC format which can be included in all three aforementioned TFCSs at a specified point.
An application of a hybrid automatic repeat request (HARQ) of the physical layer in the E-DCH has been considered. In HARQ, Node B transmits feedback in form of acknowledgment (ACK)/no acknowledgment (NACK) to notify a UE whether decoding of data packet via E-DCH was a success or not. Generally, assuming the previous transmission was unsuccessful, Node B discards previously unsuccessful decoding attempt and performs decoding operation on the retransmitted data packet. However, in HARQ, assuming the previous transmission was unsuccessful, Node B stores the unsuccessfully decoded data packet in the buffer and performs decoding operation after the retransmitted data packet with previously unsuccessfully decoded data packet are combined using various methods. By using the decoding operation of HARQ, decoding capability and efficiency is improved. The 3GPP standard provides a detailed explanation of HARQ methods.
As explained above, the transmission power of E-DPDCH corresponds to the transmitted TFC, which multiplexes data of E-DCH to E-DPDCH, and is semi-statically fixed to the relative value of DPCCH. When E-DCH packet is retransmitted, the TFC is same as the initial TFC, and as a result, the β-factor value also remains the same. By applying this scheme to E-DCH transmission, the transmission power ratio of E-DPDCH with respect to DPCCH remains same even in retransmission of data packet compared to the very first transmission of the same data packet.
However, considering transmission of E-DCH packet in HARQ, previously unsuccessfully decoded E-DCH packet is combined with retransmitted E-DCH packet and the combined E-DCH packet is decoded. Therefore, based on signal-to-interference ratio of the previous data packet, the transmission power of the retransmission packet can be controlled. For example, if the signal-to-interference ratio of the previously transmitted but unsuccessfully decoded data packet is 6 and the signal-to-interference ratio for successful decoding is 7, the retransmission of the data packet can be transmitted by lowering the transmission power to satisfy the signal-to-interference ratio of 1 (7−6=1).
On the other hand, it is possible for the communication system to experience instability if a large amount of data packets are transmitted at a same specified time. Because there is more likelihood of unsuccessful decoding of data packets, in turn causing increased number of retransmission. If the number of retransmissions increases, Node B could prohibit retransmission or reduce transmission power of the retransmitted data packet, in order to stabilize the communication system.